What causes lightning?

Lightning strikes!

Have you ever got a static electricity shock? Or seen sparks
when you take off your jumper? When lightning is made the same
thing happens, but on a much bigger scale.

How does lightning form?

Lightning is an electric current. To make this electric current,
first you need a cloud.

When the ground is hot, it heats the air above it. This warm air
rises. As the air rises, water vapour cools and forms a cloud. When
air continues to rise, the cloud gets bigger and bigger. In the
tops of the clouds, temperature is below freezing and the water
vapour turns into ice.

Now, the cloud becomes a thundercloud. Lots of small bits of ice
bump into each other as they move around. All these collisions
cause a build up of electrical charge.

Eventually, the whole cloud fills up with electrical charges.
Lighter, positively charged particles form at the top of the cloud.
Heavier, negatively charged particles sink to the bottom of the
cloud.

When the positive and negative charges grow large enough, a
giant spark - lightning - occurs between the two charges within the
cloud. This is like a static electricity sparks you see, but much
bigger.

Most lightning happens inside a cloud, but sometimes it happens
between the cloud and the ground.

A build up of positive charge builds up on the ground beneath
the cloud, attracted to the negative charge in the bottom of the
cloud. The ground's positive charge concentrates around anything
that sticks up - trees, lightning conductors, even people! The
positive charge from the ground connects with the negative charge
from the clouds and a spark of lightning strikes.